RFR: 8194743: Compiler implementation for Statements before super() [v20]

[Archie Cobbs]

> This is a first draft of a patch for JEP 447.
> 
> Summary of changes:
> 
> 1. Track when we're within a constructor "prologue" via new flag `AttrContext.ctorPrologue`
> 1. Add checks for illegal early access to `this` in constructor prologues, and update existing checks to distinguish between static context vs. constructor prologue context
> 1. Verify allowed placement of `super()`/`this()` calls via new method `Check.checkSuperInitCalls()`
> 1. Remove/refactor assumptions in several places that `super()`/`this()` was always the first statement
> 
> The changes in `Flow.java` are an example of <span>#</span>4. `Flow.FlowAnalyzer` checks for uncaught checked exceptions. For initializer blocks, this was previously done by requiring that any checked exceptions thrown be declared as thrown by all constructors containing `super()`. This list of checked exceptions was being pre-calculated before recursing into the initial constructors. This worked because initializer blocks were executed at the beginning of each initial constructor right after `super()` is called.
> 
> Now initializer blocks are traversed as each `super()` invocation is encountered, reflecting what actually happens at runtime. Similarly, final fields are marked as DA after encountering `this()`, not automatically at the beginning of those  constructors. These changes produce equivalent checks, but are compatible with the new flexibility of placement of `super()`/`this()` as well as possible future changes that could occur along these same lines.

Archie Cobbs has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 40 commits:

 - Merge branch 'master' into SuperInit
 - Merge branch 'master' into SuperInit
 - Reword new errors to use the JLS term for "explicit constructor invocation".
 - After further review, revert 599d761 to avoid mutable state lying around.
 - Address review comments relating to error messages.
 - Have RefBeforeCtorCalledError extend StaticError and customize debug strings.
 - Change variable name "statik" -> "isStatic" per review recommendation.
 - Remove obsolete flag "constructorArgs".
 - Change the error message to match the applicable spec rule.
 - Merge branch 'master' into SuperInit
 - ... and 30 more: https://git.openjdk.org/jdk/compare/e4803e0c...bbdb814f

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Changes: https://git.openjdk.org/jdk/pull/13656/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13656&range=19
  Stats: 1498 lines in 27 files changed: 1156 ins; 225 del; 117 mod
  Patch: https://git.openjdk.org/jdk/pull/13656.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13656/head:pull/13656

PR: https://git.openjdk.org/jdk/pull/13656